On-board weighing systems are known in the art. Typically, load cells, or similar load-sensing means are mounted between the truck frame and a container in which a product is placed for transport. By this means, the weight of product in the container can be determined.
This elementary weighing system suffers from a variety of deficiencies. For example the load cells are preferably sized to provide a reasonably accurate measure of the weight of material in the container, while the truck is stationary (i.e. during loading of the truck). However, during transport, the load cells can be expected to be subjected to (at least transient) loads far in excess of the "static" weight of the product in the container. Furthermore, during transport, the container will also be subjected to substantial lateral (side-to-side), and longitudinal (front to back) forces, all of which must be resisted by the load cells. These transient loads can easily result in damage to the load cell, thus degrading the accuracy of weight measurements taken using the cell.
A variety of solutions have been proposed for protecting the load cells during transport. For example, U.S. Pat. No. 3,146,839 (Carlson) discloses a weight measuring system in which load cells are positioned between the truck frame and a portion of the body so that the weight of material stored in the truck body can be measured. Additionally, a restraining system is connected between the body and frame of the truck in order to prevent excessive lateral forces being applied to the load cells. However, this restraining system does not prevent large transient vertical loads being applied to the load cells, and thus the cells must be fabricated large enough to withstand these large vertical loads.
However, fabricating the load cells sufficiently strong enough to withstand these loads reduces the sensitivity of the load cells to such an extent that the weight of a product cannot reliably be measured with a high degree of accuracy. For this reason, a weight scale based on the disclosure of U.S. Pat. No. 3,146,839 cannot be made sufficiently precise to meet the criteria of regulatory agencies for use in trade.
In an alternative system, U.S. Pat. No. 4,905,780 describes a mobile livestock weigh scale in which the weighing cage is lifted off the load cells and supported on spacer elements or blocks during transport. The frame of the weigh scale includes extendable legs which facilitate levelling of the scale. By this means, the load cells can be precise enough to be used in trade, and precise levelling of the frame prior to use ensures that the weights measured will be accurate. However, under normal conditions, proper levelling of the scale can be very difficult, and ultimately relies on the skill of the operator and the unevenness of the terrain. Thus achieving consistently accurate weight measurements from location to location is extremely difficult.
As a result of the above-described difficulties associated with measuring weight to a sufficiently high degree of accuracy for use in trade, many products, although quoted and sold by weight, are in fact measured by volume.
For example, Anhydrous Ammonia (NH.sub.3) is a liquified gas commonly used as a fertilizer. The NH.sub.3 is typically delivered to a farm in a large "nurse" truck which has one or two large pressurised tanks for storing the product. A quantity of the anhydrous ammonia is then dispensed into storage tanks from the nurse truck, for later use by the farmer. Typically, the quantity of NH.sub.3 stored on the nurse truck is substantially more than the quantity to be delivered, and thus it is necessary to accurately measure the quantity of NH.sub.3 dispensed.
While Anhydrous ammonia is typically sold by weight, the quantity of NH.sub.3 dispensed from the nurse truck is usually measured by volume, because of the aforementioned difficulties associated with obtaining a sufficiently accurate weight measurement. However, NH .sub.3 is highly corrosive, thus leading to rapid deterioration of the volumetric meters, and resultant errors on the volume measurement. For this reason, volumetric meters must be regularly serviced and recalibrated, thus leading to increased costs.